[AZ-Observing] Re: Getting started in photometry
- From: Jeff Hopkins <phxjeff@xxxxxxxxxxx>
- To: az-observing@xxxxxxxxxxxxx
- Date: Thu, 20 Nov 2003 21:43:38 -0700
Gene Lucas replied in part:
>The "standard" photomultiplier tube for astronomy is the 1P21, which is
>electrically similar to the 931 tube -- but the 1P21 is MUCH quiter
>(selected for very low noise).
Thank you for your post.
I just came in from doing some preliminary PEP observing of Algol. It
seems my photometry equipment is still working properly. I had
forgotten how rewarding this can be.
To clarify a couple of things. The 1P21 is exactly the same as a
931A. In fact I have seen better performing 931A tubes than 1P21s
(the 1P21s had probably been abused). The 1P21 is a factory selected
tube. Those that don't make the cut, but are still within tolerance
are stamped 931A.
The 1P28 is very similar to the 1P21. It has slightly higher gain,
but very much more dark current and a different window material.
While it might be useable for UBV photometry with the proper filters
(and that can be a problem), it would be much cheaper and more
efficient just to use a 1P21. The important thing is if you want to
submit or publish PEP data, it must conform to standards. With the
1P28 it will be more difficult to do that.
Note: while the PMTs are vacuum tubes they are not like normal radio
type tubes. There is no filament. They do not get hot and do not wear
out. They are fragile and can be broken. Exposure, with HV applied,
to bright light can damage them, sometimes just temporarily and
sometimes permanently. Some times a tube can be rejuvenated by
keeping it in the dark with HV on for a few days or weeks.
As for the high voltage concern, I have to laugh at that. Unless you
use a high current supply, it would be difficult to even get a tickle
from them. While typical PMT HVPS produce around 1000VDC, they are
very low current (1-10 mA). Typically the PMT takes only about 1-2 mA
(milliamp). To feel current you must have at least around 30 mA pass
through you. Touching the high voltage would likely short out the
supply and reduce the voltage close to zero. The supply described in
my book is very easy to build and very safe. It's also inexpensive.
Anyone wishing to attempt building their own photometer and has any
questions or needs any help, I'll be happy to assist. I can think of
some great projects for Thad's ATM group!
While Berry's CCD Cookbook was great many years ago, it is sadly
outdated. You can buy a used or surplus web cam for under $20 that
will be superior. The secret now is in the software. Anyone
considering the CCD photometry should check the AACSO web site.
http://www.aavso.org/observing/programs/ccd/
I asked before an will ask again if anyone is interesting in getting
a photometry group together. If anyone would like a demonstration
some evening, they are welcome to come over. Just e-mail me in
advance.
Jeff
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Jeff Hopkins
HPO SOFT
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